This past spring scientists had taken measurements along a 280-mile (450-kilometer) route across the northern part of the Beaufort Sea (map). Most of the ice, they found, was young and thin.

"With a larger part of the region now first-year ice, it is clearly more vulnerable," expedition leader and sea-ice expert Peter Wadhams, og the University of Cambridge in England, said in a statement.

The Arctic Ocean ice cover last spring was 6 feet (1.8 meters) thick, on average, indicating that it was only about a year old, the explorers said. More durable, multi-year ice, by contrast, is about nine feet (three meters) thick, according to NASA.

This relative thinness is an indication of the Arctic sea ice's poor health, said Mark Serreze, an Arctic-ice expert at the National Snow and Ice Data Center in Boulder, Colorado, in an interview with National Geographic News.

"Simply viewed, if we start out the melt season in spring with ice that is thin, it simply doesn't take as much solar energy to melt it out than if it was thicker," he said.

Also, wind and ocean currents can more easily break up thinner ice, which exposes even more ice to warmer water. And once ice is free floating, winds and currents can push the ice south into warmer waters outside the Arctic Circle, Serreze added.

Dueling Dates for Arctic Ice Melt

The new data, presented by the Catlin Arctic Survey and the international conservation group WWF, support the view that the Arctic will be ice free in the summer within about 20 years.

Most of the ice melt is expected to happen within the next ten years, Wadhams said in his statement.

Serreze's group in Boulder, though, is on record saying the Arctic's summer sea ice will fully melt around 2030. Other groups have put the ice-free date as late as 2100.